Plating is a process used by deposition of metals upon a surface from an aqueous solution (electrolyte) containing metal salts. Said process may take place electrolytically (applying an electric current) or purely as a chemical reaction (electroless plating) without applying an external current source. Chemical and electrochemical processes can be further subdivided into three different sub-groups: electrolytic plating, autocatalytic plating and ion exchange (displacement plating) plating.
Electroless plating, also known as electroless metal plating or chemical or auto-catalytic plating, involves several simultaneous reactions in an aqueous solution, which occur without use of external electrical power. The reaction is accomplished when hydrogen is released by a reducing agent, normally sodium hypophosphite, and oxidized, thus providing a negative charge on the surface of the part. The most common electroless metal plating method is electroless nickel plating, although for example silver, gold and copper layers can also be applied in this manner.
Electroless nickel plating (EN) is an auto-catalytic chemical technique used to deposit a layer of nickel-phosphorus or nickel-boron alloy on a solid substrate, such as metal, ceramic or polymer material. The process relies on the presence of a reducing agent, for example hydrated sodium hypophosphite (NaPO2H2.H2O) which reacts with the metal ions to deposit metal.
Alloys with different percentage of phosphorus are called low phosphorus, medium phosphorus (sometimes referred as mid-phosphorus) and high phosphorus. The metallurgical properties of the alloys depend on the percentage of phosphorus.
A common form of electroless nickel plating produces a nickel phosphorus alloy coating. The phosphorus content in electroless nickel coatings can range for example from 2% to 13%. It is commonly used in engineering coating applications where wear resistance, hardness and corrosion protection are required. All the Ni—P types can be applied with uniform coating thickness, also on most complicated surfaces. The wear and hardness properties of resultant coatings are greatly affected by not only the bath composition but also the deposition temperature, pH and age of the bath. Electroless nickel plating layers are known to provide extreme surface adhesion when plated properly. Electroless nickel layers are not easily solderable, nor do they seize with other metals or another electroless nickel plated work piece under pressure. Electrical resistance is higher compared to pure metal plating.
Electroless nickel plating baths are sensitive for metallic and organic impurities. Even very low levels of these impurities can cause failure in plating, such as dullness, pitting or the bath may plate out spontaneously.
Electrolytic plating, also known as electroplating, is the most widely applied use of plating technique. It requires an external power source and the plating is normally carried out with the entire surface area immersed in a liquid. The metal source consists of metal ions and possibly also metal anodes, which will be continuously dissolved as the metal plating takes place. In some cases the anode is just an inert conducting electrode made from, e.g. platinum-coated titanium or graphite (known as dimensionally stable anodes, DSA), where the metal ions are only supplied from the electrolyte which is gradually consumed and it is therefore necessary to frequently add more ions to the electrolyte (replenish).
Typical industrial electroplated coatings include hard chrome (also known as hexavalent chrome (Cr6+)), decorative chrome and various nickel coatings.
Hard chrome's main applications can be found within oil & gas industries, automotive and aerospace industries and on various industrial machinery parts. One functional disadvantage of hexavalent chromium plating is low cathode efficiency, which results in bad throwing power. Hence, the coatings become non-uniform, the coating thickness being higher at plated component edges. To overcome this problem the part may be over-plated and ground to size, or auxiliary anodes may be used around the hard-to-plate areas. Altogether, this results in high electricity consumption and cost. From a health standpoint, hexavalent chromium is the most toxic form of chromium. In the U.S. the Environmental Protection Agency (EPA) regulates it heavily. The EPA lists hexavalent chromium as a hazardous air pollutant because it is a human carcinogen, a “priority pollutant” under the Clean Water Act, and a “hazardous constituent” under the Resource Conservation and Recovery Act. Due to its low cathodic efficiency and high solution viscosity a toxic mist of water and hexavalent chromium is released from the bath. Due to significant health risks, European Union is to ban or severely restrict the use of hard chrome within its territory.
Trivalent chromium is an alternative to hexavalent chromium plating in certain applications and thicknesses, e.g. decorative plating. Plating thickness is significantly thinner than with hard chrome. Said coating thickness is limited due to strain and subsequent coating delamination generated due to absence of strain releasing cracking typical in hard chrome plating. From a health standpoint trivalent chromium is intrinsically less toxic than hexavalent chromium. The disadvantages include its limited thickness and thus wear and corrosion resistance properties, the need to use various additives to adjust the coating color, and its sensitivity with respective to metallic impurities.
Gold plating is a method of depositing a thin layer of gold typically on copper or silver. Gold can be deposited by electrolytic and electroless means. There are several standards in which gold platings can be divided, but often they are divided into pure and hard gold which can be further divided based on their pH level or whether they contain cyanide or not. The wear resistance and hardness of pure gold coating is poor, typically below 130 HV. Gold hardness properties can be improved by alloying said gold coatings with transition metals, most often using cobalt or nickel. The hardness of hard gold is between 120-300 HV. Hard gold can only be produced from acidic cyanide based baths. Cobalt or nickel hardened gold cannot be used in semiconductor industry for die bonding because they interfere with the process. European Union is however having plans to ban the use of cobalt alloys and thus, there is an identified need for improving the gold coating wear and corrosion resistance properties by other means. As improved coating wear resistance would facilitate thinner coating thicknesses without compromising on the coating lifetime, significant savings in gold material and processing costs could be achieved.
As the electrical conductivity of silver is higher than that of copper, silver is often times applied as a top coat on copper, within electronics applications. This is valid especially in high frequency applications, due to skin effect.
When the silver layer is porous or contains cracks, the underlying copper readily subjected to galvanic corrosion, flaking off the plating and exposing the copper itself; a process known as red plaque. Hence, improvements in silver-plated coating corrosion resistance would improve the coating lifetime. Moreover, improved coating wear and corrosion resistance would facilitate thinner coating thicknesses without compromising on the coating lifetime and significant savings in silver material and processing costs could be achieved.
Nickel phosphorous coatings can also be plated electrolytically. The coatings typically contain around 11-13% of phosphorous which explains the good corrosion resistance of the coatings. The hardness of electrolytic NiP coating is typically around 550-600 HV. The advantage over electroless nickel phosphorous plating is e.g. that the plating rate and thickness can be controlled easier, metal additions are not necessary because of soluble nickel anodes and the there is no plate out of nickel.
Electroplated nickel-silicon carbide composite coatings are used for example in two-stroke engines as cylinder bore coatings. The process is commonly known as Nikasil-process. The coatings generally have higher wear resistance than regular nickel coatings, but the work-piece touching the surface will wear out quickly due to the nature of the irregularly shaped SiC-particles, unless lubrication is used. The silicon carbide particles (Nikasil) co-deposited in the coating improve the plated nickel coating's affinity to oils and lubricants and thus, reduce the overall friction within the friction pair. The bath (electrolyte) SiC particle concentration is typically 40 g/l.
Electrolytic and electroless plating can take place both in acidic, neutral and alkaline conditions, having an impact on various particulate's stability therein. The lower is the particles stability in the electrolyte, the more they have tendency to agglomerate. This issue has traditionally been addressed by increasing the particle additive content and using various suitable surfactants.
Nanodiamonds can be produced by synthetic or detonation processes.
Synthetic nanodiamonds may be produced by several known methods, such as chemical vapour deposition or high pressure high temperature (HPHT) method, followed by crushing and sieving of resulting diamond particles. Such particles particle size distribution (PSD) is wide and the particle size (D50) varies from tens of nanometers to several hundreds of micron size. Nanodiamonds produced this way don't exhibit surface functionalization, nor can their surface be functionalized with covalently bound surface functions. Moreover, their shape is irregular and the particles exhibit hard edges.
Nanodiamonds produced by detonation synthesis are called detonation nanodiamonds. That is, detonation nanodiamonds originate from detonation process.
Detonation nanodiamond, also referred to as ultrananocrystalline diamond or ultradispersed diamond (UDD), is a unique nanomaterial, which can be produced in thousands of kilograms by detonation synthesis.
Detonation nanodiamonds, or nanodiamonds originating from detonation process, were first synthesized by researchers from the USSR in 1963 by explosive decomposition of high-explosive mixtures with negative oxygen balance in a non-oxidizing medium. A typical explosive mixture is a mixture of trinitrotoluene (TNT) and hexogen (RDX), a preferred weight ratio of TNT/RDX is 40/60.
As a result of the detonation synthesis, diamond-bearing soot also referred to as detonation blend is obtained. This blend comprises spherical nanodiamond particles, which typically have an average particle size of about 2 to 8 nm, and different kinds of non-diamond carbon contaminated by metals and metal oxide particles coming from the material of the detonation chamber and used explosives. The content of nanodiamonds in the detonation blend is typically between 30 and 75% by weight.
The nanodiamond-containing blends obtained from the detonation contain same hard agglomerates, typically having a diameter of above 1 mm. Such agglomerates are difficult to break. Additionally the particle size distribution of the blend is very broad, ranging typically from several to tens of microns.
The diamond carbon comprises sp3 carbon and the non-diamond carbon mainly comprises sp2 carbon species, for example carbon onion, carbon fullerene shell, amorphous carbon, graphitic carbon or any combination thereof. In addition, the nanodiamond blend contains metallic impurities originating mainly from the detonation chamber but sometimes also from the applied explosives.
There are number of processes for the purification of the detonation blends. The purification stage is considered to be the most complicated and expensive stage in the production of nanodiamonds.
For isolating the end diamond-bearing product, use is made of a complex of chemical operations directed at either dissolving or gasifying the impurities present in the material. The impurities, as a rule, are of two kinds: non-carbon (metal ions, metal oxides, salts etc.) and non-diamond forms of carbon (graphite, black, amorphous carbon).
Chemical purification techniques are based on the different stability of the diamond and non-diamond forms of carbon to oxidants. Liquid-phase oxidants offer an advantage over gas or solid systems, because they allow one to obtain higher reactant concentrations in the reaction zone and, therefore, provide high reaction rates.
Nanodiamonds have received attention due to several existing applications within for example chemo-mechanical polishing, oils and lubricants additives, various polymer mechanical and thermal composites.
The usability of the detonation nanodiamonds is based on the fact that the outer surface of detonation nanodiamond, as opposite to for example nanodiamonds derived from micron diamonds by crushing and sieving, is covered with various surface functions. Typically detonation nanodiamond surface contains mixture of oppositely charged functions and exhibits thus high agglomeration strength at low overall zeta-potential properties. With agglomeration it is meant the single nanodiamond particles tendency to form clusters of nanodiamond particles, these clusters sizing from tens of nanometers into millimeter-sized agglomerates.
Substantially mono-functionalized nanodiamond possesses, depending on the type of surface functionalization, either a highly positive or negative zeta potential value.
The zeta potential value can be related to the stability of colloidal dispersions. The zeta potential indicates the degree of repulsion between adjacent, similarly charged particles in dispersion or suspension. For molecules and particles that are small enough, a high zeta potential will confer stability, i.e., the solution or dispersion will resist aggregation. When the potential is low, attraction exceeds repulsion and the dispersion will break and flocculate. So, colloids with high zeta potential (negative or positive) are electrically stabilized while colloids with low zeta potentials tend to coagulate or flocculate. If the zeta potential is 0 to ±5 mV, the colloid is subjected to rapid coagulation or flocculation. Zeta potential values ranging from ±10 mV to ±30 mV indicate incipient instability of the colloid (dispersion), values ranging from ±30 mV to ±40 mV indicate moderate stability, values ranging from ±40 mV to ±60 mV good stability, and excellent stability is reached only with zeta potentials more than ±60 mV. One of the common ways to measure the material zeta potential is laser Doppler Micro-Electrophoresis method. An electric field is applied to a solution of molecules or a dispersion of particles, which then move with a velocity related to their zeta potential. This velocity is measured using laser interferometric technique called M3-PALS (Phase analysis Light Scattering). This enables the calculation of electrophoretic mobility, and from this the zeta potential and zeta potential distribution.
Several methods for functionalizing detonation nanodiamonds with different functional groups have been developed. Typical functionalized nanodiamonds are hydrogenated nanodiamonds, carboxylated nanodiamonds, hydroxylated nanodiamonds and amino-functionalized nanodiamonds, For example, PCT/FI2014/050290 discloses a method for producing zeta negative nanodiamond dispersion and zeta negative nanodiamond dispersion, PCT/FI2014/050434 discloses zeta positive hydrogenated nanodiamond powder, zeta positive single digit hydrogenated nanodiamond dispersions and methods for producing the same, and PCT/FI2014/051018 discloses zeta positive amino-functionalized nanodiamond powder, zeta positive amino-functionalized nanodiamond dispersion and methods for producing the same.
In the recent years nanodiamonds have received more attention in the field of electroless plating in attempts to increase for example wear of the electrolessly plated metal coating.
KR 100795166 B1 discloses electroless coating method using a nanodiamond powder solution improving hardness, wear resistance, and corrosion resistance of a metal. The method includes i) inserting nanodiamond powder to water at room temperature; ii) dispersing the nanodiamond powder solution by using ultrasonication; and iii) inserting the dispersed nanodiamond powder solution into an electroless nickel coating solution by utlrasonication.
WO 2011/089933 discloses a process for producing a composite plating solution and depositing diamond microparticles in a metal plating film to impart functions such as abrasion resistance. Diamond microparticles having anionic functional groups, such as COOH, are dispersed together with an ionic or nonionic surfactant as dispersing agent to prepare dispersion, and the dispersion is added to a metal plating solution.
EP 1288162 A2 discloses metal plating solution comprising detonation nanodiamonds and a cationic surface-active agent. The detonation nanodiamonds have a large amount of negatively charged functional groups on the nanodiamond particle surface. The cationic surface-active agent is attracted by the negatively charged functional group on the nanodiamond surface, and thus stabilizing the solution.
Based on above disclosure, there is still a need for a more efficient and economical electroless and electrolytic plating methods easy to use and resulting to a metallic coating containing detonation nanodiamonds having improved mechanical, corrosion and thermal properties.